Strathclyde Police investigated complaints that Boruc, 26, had angered a section of the crowd with his behaviour and they submitted a report to the procurator fiscal.

However, as an alternative to prosecution, Boruc was cautioned. That does not leave him with a criminal record, although the information about the caution will be retained.

The Crown Office said his actions "provoked alarm and crowd trouble and as such constituted a breach of the peace".

On Saturday, it said it had taken action "based on an assessment of behaviour, not one single act, which appeared to be directed towards the crowd which was being incited by that behaviour and which caused the police to intervene and calm the crowd".

The Crown Office said a report had been sent to the fiscal as there were "clear guidelines" on the need for responsible behaviour from players taking part in such fixtures.

"The procurator fiscal concluded that the effect of the behaviour of the player on the crowd was such as to require the consideration of criminal proceedings," it said.

Witness statements and the video of the crowd's reactions were examined, but the incident itself was not caught on camera.

Religious significance

"The procurator fiscal concluded that it was necessary to bring clearly to the player's attention that conduct which involved gesticulating at and incensing the crowd at a football match amounted to the offence of breach of the peace and was unacceptable," said the Crown Office.

"The procurator fiscal took the view that in the circumstances criminal proceedings were not necessary on this occasion, which is why it was dealt with by way of an alternative to prosecution."

However, Peter Kearney, spokesman for the Catholic Church, said the move to caution Boruc was "regrettable".

He said other actions could not be defended, but that a gesture of religious significance should not be considered offensive.

"It's a worrying and alarming development, especially since the sign of the cross is globally accepted as a gesture of religious reverence," he said.

"It's also very common in international football and was commonplace throughout the World Cup.

"It is extremely regrettable that Scotland seems to have made itself one of the few countries in the world where this simply religious gesture is considered an offence."

What he was, was deliberately provocative and completely done to wind up the fans, as if the fixture isn't volatile enough

Stephen SmithRangers Supporters Group

Nationalist leader Alex Salmond also criticised the decision to caution the player.

"The procurator fiscal has taken leave of their senses. I will be demanding an explanation for this," he said.

He said the "ludicrous" move was the type of action which brought the law and legal system into disrepute.

"The procurator fiscal and the Crown Office is acting in a way that will inflame rather than reduce religious antagonism," he said.

Liberal Democrat MSP Donald Gorrie said he felt the situation could have been addressed by talking to Mr Boruc privately.

He added: "I think they were wrong to focus on the crossing rather than the whole performance, as described to me by quite sensible people who were definitely wound up by it as they were intended to be, and he shouldn't do that."

'Set an example'

The goalkeeper, who played during this year's World Cup in Germany, signed for Celtic from Legia Warsaw in July last year.

A spokesman for Celtic said the club was "currently assessing this issue".

"The club has arranged to meet with Strathclyde Police and our supporters' representatives to discuss the matter further."

Rangers Supporters Group said it was "disappointing" Celtic did not take action against the player after the game.

Stephen Smith, spokesman for the group, added: "Professional footballers are meant to set an example. What he was, was deliberately provocative and completely done to wind up the fans, as if the fixture isn't volatile enough."

National plan

The fixture takes place in a charged atmosphere and has produced no shortage of controversy down the years.

Last year 12 people were arrested for acts of "religious prejudice" after an Old Firm match during which Rangers player Fernando Ricksen was struck by an object.

The match took place days after an "historic" summit on sectarianism where it was agreed to work on a national plan to tackle the problem.

That event was attended by both Celtic and Rangers, who later in the year launched a project to tackle bigotry and sectarianism in the west of Scotland.